Modern vehicle engines generally require a heat transfer fluid (liquid coolant) to provide long-lasting, year-round protection of their cooling systems. The primary requirements of the heat transfer fluids are that they provide efficient heat transfer to control and maintain engine temperature for efficient fuel economy and lubrication, and prevent engine failures due to freeze-up, boiling-over, or over-heating. An additional key requirement of a heat transfer fluid is that it provides corrosion protection of all cooling system metals over a wide range of temperature and operating conditions. Beyond metal protection, corrosion protection helps the heat transfer fluid to fulfill its primary function of transferring excess heat from the engine to the radiator for dissipation.
A typical modern automotive cooling system contains various components to fulfill its design functions. Particularly, an automotive cooling system can contain the following components: an engine, a radiator, a coolant pump, a fan, a heater core, heater hose, heater control valve, thermostat, radiator hose, and overflow reservoir. Additional components may be incorporated, such as a transmission oil cooler and/or an engine oil cooler in some diesel powered or high performance vehicles, a heat sink in gas-electric hybrid vehicles, and an intercooler in some vehicles containing a turbocharged engine. Many different kinds of metals are typically used to fabricate the various parts of cooling system components. For example, cast iron and cast aluminum alloys may be used for cylinder blocks, cylinder heads, intake manifolds, coolant pumps, and power electronic device enclosures. Wrought aluminum and copper alloys may be used for radiators and heater cores. Solders may be used to join the components of brass or copper radiators or heater cores. Steel is frequently used for cylinder head gaskets and for small components such as freeze plugs, coolant pump housing enclosures, and coolant pump impellers. Copper alloys are often used in thermostats.
Many types of problems can be encountered in engine cooling systems including corrosion. Common corrosion-related problems in automotive cooling systems include (1) cavitation corrosion and rusting of the cylinder heads and cylinder blocks; (2) seal leakage, bellows seal failure, and cavitation corrosion in water pumps; (3) solder bloom, scale and deposit formation, and pitting in radiators and heater cores; (4) thermostat sticking; and (5) crevice corrosion at hose necks. In addition, erosion-corrosion, galvanic corrosion, under-deposit corrosion, and stray-current corrosion can occur at susceptible locations and conditions in cooling systems.
To ensure long service life and to fulfill their design functions, metal components used in automotive cooling systems have to be protected from corrosion by the heat transfer fluid. In addition, the heat transfer fluid should be compatible with non-metals (such as hoses, gaskets and plastics) used in cooling systems. Excessive corrosion or degradation of material used in the cooling system can lead to a substantial reduction in the strength of a material or component, to a loss of heat transfer fluid from the system, and to the subsequent malfunction of one or more of the cooling system components. All of these events may result in engine failure. Furthermore, it should be noted that even relatively mild corrosion can result in the formation of corrosion products that can form scales or deposits on heat transfer surfaces. These scales or deposits can greatly reduce heat transfer rate. Excessive scale or corrosion product deposition can also lead to restriction of heat transfer fluid flow in the radiator and heater core tubes, even plugging the heater core and/or radiator. Substantial heat transfer rate reduction and flow restriction of the heat transfer fluid can lead to overheating of the engine.
In addition to providing reliable corrosion protection for various metallic components in the cooling systems, a heat transfer fluid should also have following properties to fulfill its requirements for use as a year-round functional fluid for a vehicle: high thermal conductivity, high heat capacity or high specific heat, good fluidity within the temperature range of use, high boiling point, low freezing point, low viscosity, low toxicity and safe to use, cost effective and have adequate supply, chemically stable over the temperature and conditions of use, low foaming tendency, good material compatibility, i.e., does not corrode, erode or degrade system materials, including both metallic and nonmetallic materials.
To satisfy the customer's desire for more power, comfort and safety, and to meet the need of lower fuel consumption and reduced exhaust emission, new vehicle technologies are being developed constantly. Extensive efforts are being devoted to research to develop new and more environmentally friendly propulsion technologies, such as fuel cell and petroleum-hybrid electric power, and new material technologies, new cost effective manufacturing technologies, and to explore new methods to increase the use of lighter metals and/or materials. New heat transfer fluid technologies are often required to satisfy the needs of the new cooling systems or to improve heat transfer fluid performance, such as improved corrosion protection.
Improvements in coolant corrosion protection of metals are required to enhance corrosion protection to meet the needs of the new cooling systems. Particularly, there is an ongoing need for improvement of the corrosion protection performance of heat transfer fluids for use in cooling systems containing heat exchangers produced by controlled atmosphere brazing (CAB) process, and corrosion protection at high temperature.